Chemical hydride ammonia borane (AB, NH₃BH₃) draws attentions as a hydrogen storage medium for its high hydrogen capacity (19.6 wt %) and good thermal stability at ambient environments. However, high hydrogen generation temperatures and slow kinetics limit AB practical applications. One way to overcome the obstacles is the nanoconfinement effect: AB incorporated with porous materials has facilitated dehydrogenation process. However, the mechanism is still under debate, and several factors have been proposed, like hydride nanosize or catalytic environments controlled/provided by microporous supports. In this research, metal–organic frameworks (MOFs) of Cu-BDC (BDC = benzenedicarboxylate) with/without manipulated active open metal sites by solvent removal/capping are applied for AB thermolysis via nanoconfinement. Both AB@MOFs show the same dehydrogenation peaked temperature regardless of catalytic environments, strengthening the theory that high surface tension from hydride nanosize controlled by MOF microporosity results in reduced dehydrogenation temperature. In addition, compared to solvent-capped MOFs, Cu-BDC with copper open metal sites eliminates byproduct emission and hence increases hydrogen yield from AB and also decreases dehydrogenation activation energy considerably. However, short cycle life due to copper reduction is observed in desolvated Cu-BDC, while lowered dehydrogenation temperature can be kept in solvated MOFs. In general, we clarify possible mechanisms and factors to hydride nanoconfinement and catalysis that improve AB dehydrogenation temperature and kinetics and provide new strategies for future hydride composite materials design.

中文翻译：

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洞察金属有机骨架中开放金属位点对纳米约束作用下氢化物脱氢的催化作用

化学氢化物氨硼烷（AB，NH ₃ BH ₃）作为储氢介质因其高氢气容量（19.6 wt％）和在周围环境中的良好热稳定性而备受关注。但是，高的氢气生成温度和缓慢的动力学限制了AB的实际应用。克服障碍的一种方法是纳米约束效应：与多孔材料结合的AB促进了脱氢过程。然而，该机理仍在争论中，并且已经提出了若干因素，例如氢化物纳米尺寸或由微孔载体控制/提供的催化环境。在这项研究中，Cu / BDC（BDC =苯二甲酸）的金属-有机骨架（MOFs）通过/通过脱除/加帽操作有无活性的开放金属位点被用于纳米约束的AB热解中。无论催化环境如何，两种AB @ MOF均显示相同的脱氢峰温度，从而加强了以下理论：受MOF微孔控制的氢化物纳米尺寸的高表面张力会导致脱氢温度降低。另外，与溶剂封端的MOF相比，具有铜金属开孔位的Cu-BDC消除了副产物的排放，因此增加了AB的氢产率，并且还大大降低了脱氢活化能。但是，在脱溶剂的Cu-BDC中观察到由于铜还原而导致的短循环寿命，而在溶剂化的MOF中可以保持较低的脱氢温度。一般而言，我们阐明了氢化物纳米限制和催化作用的可能机理和因素，这些机制和因素可改善AB脱氢温度和动力学，并为未来的氢化物复合材料设计提供新的策略。